1. Field
The present invention relates generally to communication, and more specifically to techniques for broadcasting data in a multi-antenna communication system.
2. Background
A multi-antenna communication system employs multiple transmit antennas at a transmitting entity and one or more receive antennas at a receiving entity for data transmission. The multi-antenna communication system may thus be a multiple-input multiple-output (MIMO) system or a multiple-input single-output (MISO) system. A MIMO system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit antennas and the NR receive antennas may be decomposed into NS spatial channels, where NS≦min{NT, NR}. The NS spatial channels may be used to transmit data in a manner to achieve greater reliability and/or higher overall throughput. A MISO system employs multiple (NT) transmit antennas and a single receive antenna for data transmission. A MISO channel formed by the NT transmit antennas and the single receive antenna is composed of a single spatial channel.
An access point in a MIMO system may broadcast data to a number of multi-antenna user terminals, which may be distributed throughout the coverage area of the access point. A different MIMO channel is formed between the access point and each of these user terminals. Each MIMO channel may experience different channel conditions (e.g., different fading, multipath, and interference effects). Consequently, the spatial channels of each MIMO channel may achieve different signal-to-noise-and-interference ratios (SNRs). The SNR of a spatial channel determines its transmission capacity, which is typically quantified by a particular data rate that may be reliably transmitted on the spatial channel. For a time variant MIMO channel, the channel condition changes over time and the SNR of each spatial channel also changes over time.
A broadcast transmission is a data transmission that is intended to be received by any number of user terminals in the system, instead of a specific user terminal. A broadcast transmission is typically encoded and transmitted in a manner to achieve a specified quality of service (QoS). This quality of service may be quantified, for example, by error free reception of the broadcast transmission by a particular percentage (e.g., 99.9%) of the user terminals within a given broadcast coverage area at any given moment. Equivalently, the quality of service may be quantified by an “outage” probability, which is the percentage (e.g., 0.1%) of the user terminals within the broadcast coverage area that cannot correctly decode the broadcast transmission.
The broadcast transmission observes an ensemble of MIMO channels for an ensemble of user terminals in the broadcast coverage area. The MIMO channel for each user terminal may be random with respect to the MIMO channels for other user terminals. Furthermore, the MIMO channels for the user terminals may vary over time. To ensure that the broadcast transmission can meet the specified quality of service, the data rate for the broadcast transmission is typically selected to be sufficiently low so that the broadcast transmission can be reliably decoded even by the user terminal with the worst channel condition (i.e., the worst-case user terminal). The broadcast performance for such a system would then be dictated by the expected worst-case channel condition for all of the user terminals in the system. A similar phenomenon occurs for a MISO system.
There is therefore a need in the art for techniques to more efficiently broadcast data in a multi-antenna communication system.